Conventional needles have long been used to deliver drugs and other substances to humans and animals through the skin, and considerable effort has been made to achieve reproducible and efficacious delivery through the skin while reducing or eliminating the pain associated with conventional needles. Certain transdermal delivery systems eliminate needles entirely, and rely on chemical mediators or external driving forces such as iontophoretic currents or sonophoresis to breach the stratum corneum painlessly and deliver substances through the skin. However, such transdermal delivery systems are not sufficiently reproducible and give variable clinical results.
Mechanical breach of the stratum corneum is still believed to be the most reproducible method of administration of substances through the skin, and it provides the greatest degree of control and reliability. Intramuscular (IM) and subcutaneous (SC) injections are the most commonly used routes of administration. The dermis lies beneath the stratum corneum and epidermis, beginning at a depth of about 60-120 μm below the skin surface in humans, and is approximately 1-2 mm thick. However, intradermal (ID) injection is rarely used due to the difficulty of correct needle placement in the intradermal space, the difficulty of maintaining placement of the needle in the intradermal space, and a lack of information and knowledge of the pharmacokinetic profiles for many drugs delivered ID. In addition, little is known about fluid absorption limits in dermal tissue and the effect of depot time on drug stability. However, ID administration of drugs and other substances may have several advantages. The intradermal space is close to the capillary bed to allow for absorption and systemic distribution of the substance but is above the peripheral nerve net which may reduce or eliminate injection pain. In addition, there are more suitable and accessible ID injection sites available for a patient as compared to currently recommended SC administration sites (essentially limited to the abdomen and thigh).
Recent advances in needle design have reduced the pain associated with injections. Smaller gauge and sharper needles reduce tissue damage and therefore decrease the amount of inflammatory mediators released. Of particular interest in this regard are microneedles, which are typically less than 0.2 mm in width and less than 2 mm in length. They are usually fabricated from silicon, plastic or metal and may be hollow for delivery or sampling of substances through a lumen (see, for example, U.S. Pat. No. 3,964,482; U.S. Pat. No. 5,250,023; U.S. Pat. No. 5,876,582; U.S. Pat. No. 5,591,139; U.S. Pat. No. 5,801,057; U.S. Pat. No. 5,928,207; WO 96/17648) or the needles may be solid (see, for example, U.S. Pat. No. 5,879,326; WO 96/37256). By selecting an appropriate needle length, the depth of penetration of the microneedle can be controlled to avoid the peripheral nerve net of the skin and reduce or eliminate the sensation of pain. The extremely small diameter of the microneedle and its sharpness also contribute to reduced sensation during the injection. Microneedles are known to mechanically porate the stratum corneum and enhance skin permeability (U.S. Pat. No. 5,003,987). However, the present inventors have found that, in the case of microneedles, breaching the stratum corneum alone is not sufficient for clinically efficacious intradermal delivery of substances. That is, other factors affect the ability to deliver substances intradermally via small gauge needles in a manner which produces a clinically useful response to the substance.
U.S. Pat. No. 5,848,991 describes devices for the controlled delivery of drugs to a limited depth in the skin corresponding to about 0.3-3.0 mm and suggests that such devices are useful for delivery of a variety of drugs, including hormones. U.S. Pat. No. 5,957,895 also describes a device for the controlled delivery of drugs wherein the needle may penetrate the skin to a depth of 3 mm or less. The fluid in the pressurized reservoir of the device is gradually discharged under gas pressure through the needle over a predetermined interval, e.g., a solution of insulin delivered over 24 hrs. Neither of these patents indicates that delivery using the devices produces a clinically useful response. Kaushik, et al. have described delivery of insulin into the skin of diabetic rats via microneedles with a detectable reduction in blood glucose levels. These authors do not disclose the depth of penetration of the microneedles nor do they report any results suggesting a clinically useful glucose response using this method of administration. Further, there is no evidence of accurate or reproducible volume of delivery using such a device. WO 99/64580 suggests that substances may be delivered into skin via microneedles at clinically relevant rates. However, it fails to appreciate that clinical efficacy is dependent upon both accurate, quantitative, and reproducible delivery of a volume or mass of drug substance and the pharmacokinetic uptake and distribution of that substance from the dermal tissue.